Method of caustic recovery in soap manufacture



Aug. 11, 1959 R. v. OWEN ETAL 2,899,450

METHOD OF CAUSTIC RECOVERY IN SOAP MANUFACTURE Filed Dec. 6,. 1955 E v is Sheets-Sheet 1 m vm ro/ss I Roma. Vmqm-r Dw'an NORMAN Austn'r Hum-CAMPDELL,BRUMBAUGH, FREE no Grows:

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Aug. 11, 1959 R. v. OWEN ETAL 2,899,450

METHOD OF CAUSTIQ RECOVERY IN SOAP MANUFACTURE Filed Dec. 6. 1955 3Sheets-Sheet 2 /Nl//Y7OR$ RONALD \lmcan'r wen Noam/ N ALBERT HURTCAMPBELL, BauMBAuGI-l, FRE AND QRAVB Aflav-neqs Aug. 11,1959 R. v. OWENET AL METHOD OF CAUSTIC RECOVERY IN SOAP MANUFACTURE Filed Dec. 6. i9553 Sheets-Sheet 3 M. 1 "a t u w A w r? RR N is Mm NW A um "FR 0 RN m m Ac nite States Pate METHOD OF CAUSTIC RECOVERY IN SOAP MANUFACTURE RonaldVincent Owen, Sale, and Norman Albert Hurt, Lymm, England, assignors toLever Brothers Company, New York, N.Y., a corporation of MaineApplication December 6, 1955, Serial No. 551,449

Claims priority, application Great Britain December 8, 1954 8 Claims.(Cl. 260-418) This invention relates to improvements in the manufactureof soap, and in particular to neutralizing the free caustic alkali incrude lye formed in soap-making processes.

In the manufacture of soap, alkali such as caustic soda is generallyemployed to saponify vegetable and animal oils, hereinafter referred toas fatty oils. saponification of the fatty oils results in a mixture ofcrude soap and aqueous liquor containing glycerine, any unreacted fattyoil and caustic alkali as well as colouring matter and other impurities.This mixture of crude soap and aqueous liquor is generally washed, forexample with an aqueous solution of brine, and then allowed to settleinto an upper layer of grained soap and a lower aqueous layer, the lye.A number of successive washes may be applied, and each of the resultantsettled lyes withdrawn from the system. Besides brine (or otherelectrolyte used in washing the crude soap), glycerine and somedissolved soap, the lye also contains impurities washed out of the soapand free caustic alkali present from the saponification stage. Theconcentration of free caustic alkali in the spent lye may vary accordingto the degree of control exercised in the soap-making process. Forexample, spent lye from a continuous soap-making process may contain agreater proportion of caustic alkali than spent lye from a batchprocess. In continuous soap-making processes, it may be practicable, forreasons of economy of space and time, to employ a greater excess ofcaustic alkali. In a batch process, in which more time may be available,completion of the saponification reaction may be obtained with arelatively slight excess of caustic alkali.

It has been the practice to neutralize the free alkali in the lye byagitating the spent lye with acid oils, that is fatty oils containing ahigh proportion of free fatty acid. The alkali reacts readily with thefatty acids to form soap, but does not react, or only reacts to aninsignificant degree, with the glycerides (the neutral oil) of the acidoil. For this reason it has been necessary in such a process of spentlye neutralisation to employ a large bulk of material, a substantialamount of which was never utilised in the process. A

The soap formed by neutralising spent lye with acid oils is generally ofa poor quality containing most of the colouring matter and otherimpurities present in the lye. The acid oils may be part of the chargeundergoing saponification in a soap-making process. When substantiallyall the free fatty acids in the acid oils have reacted with the freealkali in the spent lye, the resultant mixture of soap, glycerides andany remaining free fatty acid, after separation from the lye andremoval, as best possible, of impurities and colouring matter containedin this mixture, is usually returned to the soap-making process.

It has now been found that glycerides in fatty oils will react readilywith free caustic alkali in spent lye under conditions of vigorous andsustained agitation, yielding a soap of satisfactory quality.

Accordingly, the present invention provides a process of neutralisingthe free caustic alkali in spent lye from Patented Aug.. 11, 1959 asoap-making process, in which a stream of the spent lye is mixed with astream of fatty oil containing free fatty acid, if any, in an amountinsufficient to neutralise all the free caustic alkali in the lye, andthe resultant mixture is vigorously agitated, and the vigorous agitationis continued until substantially all the free caustic alkali has reactedwith the glycerides in the fatty oil.

Suitable amounts of soap aid a saponification reaction betweenglycerides and free caustic alkali by promoting more intimate contactbetween the reactants. Although spent lye from a soap-making processalways contains some soap, it has been found advantageous in the presentprocess to increase the concentration of soap. Accordingly, it ispreferred to add an additional amount of soap to the reactants of thepresent process. The additional amount of soap may be added to eitherthe lye or the fatty oil, but preferably it is added to a mixture ofthese two reactants. V

The soapto be added may be grained soap from a soapmaking process, or itmay be part of the soap formed during the process of the invention. Soapfrom any other source, such as neat soap or soap chips, may also beadded. Generally, the electrolyte content of the lye is such that theadditional soap added to the process is present in a finely grainedcondition.

According to the present invention it is preferred to use for theneutralisation of the caustic alkali in the spent lye a portion of theoil charge undergoing saponification in the soap-making process fromwhich the lye derives. The oil charge may be any glyceride oil, or itmay contain free fatty acids, for example, as in most commercial oilsused for soap-making; acid oils resulting from the splitting by mineralacid of soapstock formed in the alkali refining of oils may also beused. Saponification of the oil proceeds after some of the free alkalihas been neutralised by any free fatty acid present in the oil.

The rate of saponification differs for different oils. Also, the totalreaction time for a given quantity of oil and alkali varies inverselywith the temperature, .and to some extent with the intensity ofagitation which is used during the process, but in general willnot'exceed 3 hours. The agitation must, however, always be suflicientlyintense to provide for intimate mixing of the reactants. Under optimumconditions only minutes or less are required to carry the process tocompletion. It may, therefore, be preferable to reduce the space/timerequirements of the reaction by providing the most intense agitationthat can be applied economically.

For most fatty oils a temperature of reaction in the range of 90 to C.is preferred. Good results have been obtained at temperatures of C. andhigher. At temperatures lower than 85 C. the process may becomeuneconomic as the rate of reaction slows down. When using lye directlyfrom a soap-making process, it is preferred, so as to conserve heat, tocarry out the process of the invention as soon as possible afterseparating washed soap from the lye.

The time of reaction may be reduced by using an excess of the fatty oilover the stoichiometric proportion required for the reaction to proceedto substantial completion, preferably an excess of at least 10% byweight, based on the weight of free caustic alkali in the lye;

The process of the invention is suitable for working in conjunction witha batch or a continuous process of soap-making. It is particularlyadapted for use with the latter because the crude spent lyes obtainedfrom a continuous soap-making process canbe used to saponifycontinuously both the fatty acids and the glycerides in a part of thefat charge being fed to sucha process.

Yielding relatively pure material, the process of the invention, whencarriedout in'conjunction with a continuous soap-making process,therefore allows nearly completely saponified material to be directlyfed back into the saponification stage of the soap-making process. Thisis a great advance over the conventional methods, Where a much largerbulk of ,onlypartly saponified material had to be cleanedfbefore beingincorporated in othersoaps.

.For economic working it is preferred to agitate the exhausted spent lyefrom the process of the invention,

.that. is lyein which the'free caustic alkalihas been neutralised, witha small proportion of acid oil. The acid oil facilitates the substantialremoval of any dissolved soap from the exhausted spent lyefrom which itseparates after the agitation as a soap/ oil emulsion. A batch of acidoil may be usedto treat successive .quantitiesof exhausted spent lyeuntil the batch of oil itself becomes saturated with soap. Aftersaturation the oil may be cleaned, and then the clean soap orsoap/oilemulsion may be added to the soap-making process. This soapextraction step may be carried out in a continuous .man-

her, and in this case a'batch of acidoilis-preferably recycled until itbecomes saturated with soap washed out from the exhausted spent lye. Atthis stage, any free alkalinity in the exhaustedspent lye due to, forexample, sodium carbonate, may also be neutralised by the free fattyacids in the acid oil.

The invention will now be described with reference to the accompanyingdrawings in which:

Fig. 1 shows diagrammatically an arrangement of apparatus for carryingout the invention,

Fig. 2 shows diagrammatically another arrangement of apparatus forcarrying out theinvention,

Fig. 3 shows diagrammatically yet another arrangement of apparatus forcarrying out the invention, and

Fig. 4 shows a cross section on the line IVIV of Fig. 1.

Referring to Fig. l, a vessel 1, fitted with a weir 2, is provided withthree supply conduits 3, 4 and 5. The vessell is connected by conduit 6to the bottom of the first of three vessels 7a, 7b and 70, each fittedrespectively with a pair of vertical baflles 8a, 8b and'8c, and a pairof impellers 9a, 9b and 9c on vertical shafts110a, 10b and 100, drivenby motors 11a, 11b and 11c. Conduits 12a and 12b,.provided with vents13a and 13b to prevent syphoning, connect respectively the top ofvessels 7a-and 7b withthe bottom of vessels 7b and 7c. The vessels arearranged stepwise so that liquid can flow under the influence of gravityfrom the top of one vessel to the bottom of the next. Vessel 7c isprovided with a recessed compartment 14, in which rides a float 15. This-float is coupled by lever arms 16a and 16b to a butterfly valve 17,located in the suction conduit 18. ofpump "19. Conduit 18 isconnected-to the top of vessel 70.

A delivery conduit20 of pump 19 terminates in a distributing manifold 21located within a settling 'vessel 22. This vessel 22 may be of anysuitable shape including that l of a shallow tray. It is fitted withmeans for controlling automatically the lye and soap outflows, the meansconsisting of a float 23 adapted to be supported by the lye at theinterface of the lye and the soap. The float has an upper guide rod 24passing loosely, between stops 25, through a guide 26. A lower. guiderod 27 passes through a lye outlet orifice and collar 28. Near its lowerend guide rod 27 carries a small inverted cone 29 adapted to ride in thelye outlet orifice and collar 28. This lye outlet orifice and collar 28communicates with a conduit 30 which is provided with a vent 31,-terminating, as

shown, above the level of the soap in the settling vessel 22 to preventvortex formation impeding the smooth movement of theifloat 23. Near thetop of vessel 22, and

on. the side opposite the .distributing manifold 21, a slot 32is'provided toacLas a weir. 4A gutter 33 surrounds slot.32 and isprovided with a short slot 34 which is in .turn surrounded by a shortgutter 35. The arrangement .of slots and'gutters is further illustratedin Fig. 4, which shows a cross section on. theline IV-.IV of. Fig, 1. A

vertical partition 36 divides gutter 35 into two equally sizedcompartments, and slot 34 into two halves. A plate 37 is movable bymeans, not shown, in a lower horizontal slide 38 and an upper horizontalslide, not shown, the slides being located respectively below and abovethe slot 34. One of the compartments in gutter 35 communicates withvessel 1 through conduit 5. The other compartment communicates with thesaponification vessel of a soapmaking process (not shown) throughconduit 39.

The operation of the above described arrangement is as follows:

Crude spent lye from a soap-making process enters vessel 1 throughconduit 4 below the level of the contents of-the vessel to preventfoaming. Fatty oil, the flow of which is adjusted inthe correctproportions according to the feed rate and the alkalinity of the lye,enters through conduit 3. A portion ofthe soap formed during the processis returned through conduit 5 to increase the speed of the reaction. Themixture of lye,

oil and soap passes over baffle 2 and then under the influence ofgravity through conduit 6 to the bottom of vessel 7a. The shafts 10a,10b and in vessels 7a, 7b and 7c are driven at speeds of about 1,000revolutions per minute by motors 11a, 11b and lie respectively. Afterpassage under the influence of gravity through each of the three vesselsby way of conduits 12a and 12b respectively, the mixture, in whichsubstantially all the free caustic alkali has reacted with the fattyoil, is passed through conduit 18 to pump-19. The float/lever arm andbutterfly valve arrangement denoted by 15 to 17 controls the pumpsection and maintains the level of the reacted mixture in vessel '70 ata constant height above the inlet of conduit 18, thus avoiding an intakeof airinto the system The mixture is finally pumped through conduit 20and distributing manifold 21 to the settling vessel '22, settlingthereininto an upper layer of aqueous soap and a lower layer of spent lye. Thefloat 23 and the cone'29 within vessel 22 rise or fall with changes inthe level of the soap/ lye interface, thus opening or closing theorifice in collar 28, and keeping constant, within certain limits, thelevel of the soap/lye interface. The exhausted spent lye passes out ofvessel 22 through conduit 30 and is then treated with acid oil toextract dissolved soap. The soap from the upper layer spills throughslot 32 into gutter 33, and thence through slot 34. By adjusting theposition of plate 37 the proportions of soap being recycled to the lyeneutralisation process through conduit-Sand to the saponification stageof the soap-making process through conduit 39 may be controlled.

In practice, forexample, vessels 7a, 7b and 70, each of 500 to 1,000kgm. capacity, are suitable for continuously neutralising hot spent lyesupplied direct from a continuous soap-making plant at the rate of 1,000to 1,500 kgm. per hour. The contents of each vessel are preferablyagitated by impellers rotating at up to 1,000 revolutions per minute.However, a variety of agitators may be employed. For example,comparatively large paddles being rotated at 100 revolutions per minute,or less, may be used if suitable horizontal baflles are also provided.The spent lye from a soap-making process, that is, the lye used in thepresent process, usually has a free alkali content of from 0.5% to 1.0%,but lower or higher contents of up to 2 or 3% may be found. Theexhausted spent lye contains only about 0.02 to 0.05% of free alkaliafter treatment with commercial fatty oils containing as little as 1% offree fatty acids. The rate at which the soap from settling vessel 22 isrecycled to vessel 1 is preferably approximately equal to that at whichfresh soap is produced by the process.

The following examples illustrate the invention when carried outwith thearrangement of apparatus shown in Fig. l,

Example 1 Fatty oil charge: 3 parts of tallow, 1 part of palm kerneloil, bleached by treatment with fullers earth. Characteristics of fattyoil charge:

Free alkalinity of lye expressed as percent weight per weight (sodiumoxide) Na O:

Initial crude lye 0.64

Lye from vessel 7a 0.30

Lye from vessel 7b 0.11

Lye from vessel 70 0.04

Example 2 Fatty oil charge: 3 parts of tallow, 1 part of palm oil, 1part of palm kernel oil, bleached by treatment with fullers earth.

Characteristics of fatty oil charge:

Free fatty acid (as oleic acid) percent 1.39

Moisture do 0.5

Saponification equivalent 274.8 Flow rates:

Crude lye kilogrammes per hour 1520 Fatty oil do 80.3

Soap recycled from process do 36.6

Free alkalinity of lye expressed as percent weight per weight (sodiumoxide) Na Oz Initial crude lye 0.49

Lye from vessel 7a 0.24'

Lye from vessel 7b 0.09

Lye from vessel 7c 0.04

Example 3 Fatty oil charge (percent by weight):

70% tallow 15% soft acid oils 15% palm kernel acid oil Characteristicsof fatty oil charge:

Free fatty acid (as oleic acid) percent 22.0

Moisture do 0.7

saponification equivalent 265.5 Flow rates:

Crude lye kilogrammes per hour 1015 Fatty oil do 60.9

Soap recycled from process do 27.4

Free alkalinity of lye expressed as percent weight per weight (sodiumoxide) Na O:

Initial crude lye 0.56 Lye from vessel 7a 0.12 Lye from vessel 7b 0.05Lye from vessel 70 0.02

In an alternative arrangement shown diagrammatically in Fig. 2, a singlecylindrical vessel 40 is divided into four compartments, namelycompartments 41a, 41b and 410, which correspond to vessels 7a, 7b and 7cin Fig. 1, and a settling compartment 42. Each of the partitionsdividing the compartments 41a, 41b and 410 have an aperture provided toallow the reaction mixture to pass from one compartment to the next. Theaperture in the partition between compartment 41c and compartment 42 isin communication with a conduit and manifold distributor 43. In each ofthe compartments 41a, 41b and 410, impellers carried on inclined shafts44a, 44b and 440, as shown, are driven by electric motors 45a, 45b and450, respectively. Baffles may be provided in each of the threecompartments, if desired. The vessel 1, arrangement of gutters 33 and35, slots 32 and 34, appropriate conduits and slides, and parts withinthe settling compartment 42, are similar to those described inconnection with Figs. 1 and 4. The pump 19 is, however, arranged betweenthe Vessel 1 and compartment 41a, that is, in conduit 6 and not conduit20 of Fig. 1, which is dispensed with. In the present arrangement, theconduit 30 is partly, and collar 28 is wholly, within the compartment410.

The operation of the arrangement of apparatus shown in Fig. 2 is verysimilar to that shown in Fig. 1. However, instead of the reactionmixture passing through saponification vessels 7a, 7b and 70 under theinfluence of gravity, it is progressively forced upwards, by pump 19,from compartment 41a to 41b to 410, and thence through the manifolddistributor 43 into settling compartment 42. The presence of themanifold distributor ensures quiescent conditions in the settlingcompartment.

The rate of saponification may be increased by having a preponderance ofone or other of the reactants at the beginning and the end of thereaction. This is the principle which underlies the arrangement ofapparatus shown in Fig. 3. Referring to Fig. 3, two cylindrical vessels46 and 47 are provided With compartments 48a and 49a, and 48b and 49brespectively. Compartments 48a and 48b are provided with impellers onshafts 50a and 50b and motors 51a and 51b, respectively. Compartments49a and 491) are provided with the arrangement of float, orifice andcollar, and manifold distributor described in connection with thesettling vessel 22 of Fig. 1. As in Fig. 1 a conduit 3 supplies spentlye to a vessel 1 which is provided with a weir 2. Vessel 1 is connectedby a conduit 6 to the bottom of compartment 48a of vessel 46.Compartment 49a is provided with the same arrangement of gutters, plateand slots as is the vessel 22 of Fig. 1. It is also provided withconduits 5 and 39. Conduit 5 empties into a sump tank 52, fitted with afloat and butterfly valve arrangement similar to that fitted to vessel7c in Fig. 1. A conduit 53, which is connected to the variable orificeand collar in the bottom of compartment 49a also empties into sump tank52. Butterfly valve 17 is on a conduit 54, which includes pump 19.Conduit 54 connectssump tank 52 with a vessel 55, similar to vessel 1,and fitted with a weir 56. A conduit 4 supplies fresh oil to vessel 55.A conduit 57 connects the bottom of vessel 55 to the bottom ofcompartment 48b of vessel 47. Compartment 4% of vessel 47 is providedwith a gutter 58 for receiving material from the top of the compartment.A conduit 59 from gutter 58 feeds vessel 1.

The operation of the arrangement shown in Fig. 3 is as follows:

Crude lye is passed to vessel 1 through conduit 3, where it is mixedwith partly saponified material, obtained in a manner described below,from compartment 49b of vessel 47. The mixture, which contains an excessof lye, passes under the influence of gravity through conduit 6 tocompartment 48a of vessel 46. saponification of the fatty oil proceedsto completion in this compartment. Soap and partly spent lye separate incompartment 49a of vessel 46. The soap flows into gutter 33 and thendivides into two streams in the manner described previously. One streamof the soap is conveyed, through conduit 39, to the saponification stageof a conventional soap-making process. The other stream is recycled tothe present process through conduit 5 to sump tank 52. The partly spentlye is conveyed through conduit 53 also to sump tank 52, and theresultant mixture of partly spent lye and soap is pumped through conduit54 by pump 19 to vessel 55. Fatty oil is added to vessel 55 throughconduit 4 and the mixture, now containing an excess of fatty oil, flowsunder the influence of gravity into compartment 48b of vessel 47 throughconduit 57. In this compartment the free caustic alkali in the partlyspent lye is substantially neutralised. The mixture containing theexhausted spent lye passes into compartment 49b where it sepa 7 ratesinto .two layers. Thebottorn layer, exhausted spent lye, passes out ofthe system through conduit 30 so that it may be treated, preferablycontinuously, with acid oil, to removeanydissolved soap. The top layer,

containing partly saponified oil, flows into gutter 58 and throughconduit 59 to vessel 1, where it meets fresh crude lye. The abovedescribed cycle of operation then recommences.

We claim:

1. A process of neutralizing the freecaustic .alkaliin spent lye from asoap-making process the free caustic alkali content of said spent lyebeing ,not greater than 3%, comprising mixing a stream of, the spent lyewith a stream of fatty oil, the amountof fatty o il fed into the streamof the spent .lye being stoichiometrically at least 10% in excess of theamount of free alkali in the spent lye, vigorously agitating theresultant mixture at a temperature in the range of from 90 to 100 C. andcontinuing the vigorous agitation within said temperature range untilsubstantially all the free caustic alkali has been reacted with theglycerides in the fattyoil, separating the exhausted spent lye from thesaponified oil, and separately discharging the exhausted spent lye andsaponified oil.

2. A process of neutralizing the free caustic alkali in spent lye from asoap-making process, the free caustic alkali content of said spent lyebeing not greater than 3%, comprising mixing a stream of the spent lyewith a stream of fatty oil, the amount of fatty oil fed into the streamof spent lye being stoichiometrically at least 10% in excess of theamount of free alkali in the spent lye, adding soap to catalyze thereaction of the free caustic alkali with the glycerides in the fattyoil, vigorously agitating the resulting mixture at a temperature .in therange from 90 to 100 C. and continuing the .vigorous agitation withinsaid temperature range until substantially all the free caustic alkalihas reacted with the glycerides in the fatty oil, separating theexhausted spent lye from the saponified oil, and separately dischargingthe exhausted spent lye and saponified oil.

3. A process of neutralizing the free caustic alkali in spent lye from asoap-making process, thefree caustic alkali content of said spent lyebeing not greater than 3%, comprising mixing a stream of the spent lyewith a stream of fatty oil, the amount of fatty oil fed into the streamof spent lye being stoichiometrically at least 10% in excess of theamount of free alkali in the spent lye, passing the mixed stream ofspent lye and oil in the presence of grained out soap through a reactionvessel maintained at a temperature in the range of 90 to 100 C. in whichvigorous agitation is provided, at a rate to accomplish the requiredneutralization during the passage of the mixture of lye and oil throughthe reaction vessel,

separating the exhausted spent lye from .the saponified oil andcontinuously and separately discharging the exhausted spent lye andsaponified oil.

4. A process of neutralizing the free caustic alkali in the -freecaustic alkali-'in the lye, vigorously agitating the resulting mixtureand continuingithe vigorous agitation until the fatty oil is fullysaponified, continuously separatingthe spent lye from the saponifiedoil, the free caustic alkali content of said spent -lye being notgreater than 3%, continuously and separately discharging .thesaponified-oil and-the spent lye, -mixing a stream -.of the spent lyewith the stream of fatty oil, the amountof fatty oil fed into the streamof spent lye being stoichiometrically at least 10% in excess of theamountof free caustic alkali in the spent lye, vigorouslyagitating theresulting mixture at a-temperature in the range of to v C. andcontinuing the -vigorous agitation within said temperature range untilsubstantially all the free caustic alkali has reacted with theglycerides in .the fatty oil, separating the exhausted spent lye fromthe saponified oil and separately discharging the exhausted spent lyeand saponified oil.

5. A process of neutralizing the free caustic alkali in spent lye from asoap-making process, the free caustic alkali content of said spent lyebeing not greater than 3%, comprising mixing a stream of the spent lyewith a stream of fatty oil, the amount of fatty oil fed into the streamof the lye being stoichiometrically at least 10% in excess of the amountof free alkali in the spent lye, adding soap to catalyze the reaction ofthe free caustic alkali with the glycerides in the fatty oil, vigorouslyagitating the resulting mixture at a temperature in the range of from 90to 100 C., and continuing the vigorous agitation within said temperaturerange until substantially all the free caustic alkali has reacted withthe glycerides in the fatty oil, separating the exhausted spent lye fromthe saponified oil, separately discharging the saponified oil and theexhausted spent lye, and extracting from the exhausted spent lyedissolved soap by treatment with a small amount of acid oil.

6. The process according to claim 1 in which the free caustic alkalicontent of the spent lye is from 0.5% to 1.0%.

7. A process according to claim 2, in which the added soap is a recycledpart of the soap formed during the process.

8. A process according to claim 5, in which a small amount of acid oilis used to treat successive quantities of the exhausted spent lye untilthe amount of acid oil is saturated with soap.

References vCited in the file of this patent UNITED STATES PATENTS2,232,544 Lorenz Feb. 18, 1941 2,300,750 Scott Nov. 3, 1942 2,727,915Palmquist Dec. 20, 1955 OTHER REFERENCES Davidsohn et al.: SoapManufacture, vol. 1 (copyright 1953), pp. 344-345.

1. A PROCESS OF NEUTRALIZING THE FREE CAUSTIC ALKALI IN SPENT LYE FROM ASOAP-MAKING PROCEES, THE FREE CAUSTIC ALKALI CONTENT OF SAID SPENT LYEBEING NOT GREATER THAN 3%, COMPRISING MIXING A STREAM OF THE SPENT LYEWITH A STREAM OF FATTY OIL, THE AMOUNT OF FATTY OIL FED INTO THE STREAMOF THE SPENT LYE BEING STOICHIOMETRICALLY AT LEAST 10% IN EXCESS OF THEAMOUNT OF FREE ALKALI IN THE SPENT LYE, VIGOROUSLY AGITATING THERESULTANT MIXTURE AT A TEMPERATURE IN THE RANGE OF FROM 90 TO 100* C.AND CONTINUING THE VIGOROUS AGITATION WITHIN SAID TEMPERATURE RANGEUNTIL SUBSTANTIALLY ALL THE FREE CAUSTIC ALKALI HAS BEEN REACTED WITHTHE GLYCERIDES IN THE FATTY OIL, SEPARATING THE EXHAUSTED SPENT LYE FROMTHE SAPONIFIED OIL, AND SEPARATELY DISCHARGING THE EXHAUSTED SPENT LYEAND SAPONIFIED OIL.